Replacement of electrical contact brushes in electric machines such as motors and generators is usually a difficult and time consuming task. Conventionally, brush holding structures are provided either in the interior of the end plate of the motor or on the inside of the motor housing. In either event, the motor must be partially disassembled to replace brushes. This is a time consuming and difficult task, particularly when the motor must be removed from the assembly before it can be disassembled to replace brushes. Such disassembly is necessary in many applications, and in particular where the motor is installed on an electrically driven vehicle. In addition, holding the brushes and brush box clear of the armature contact surface while a armature is being installed in the motor, or while the brushes are being installed in the motor, is important, to avoid scratching the armature contact surface. Such a surface is known as a commutator in the case of a D.C. machine, and as a slip ring in A.C. machines. Any damage to the commutator or slip ring tends to magnify itself, causing accelerated wear of both the brushes and the armature contact surface. There are two basic, nonexclusive, mechanisms by which a scratch in a commutator bar or slip ring may accelerate wear to both brushes and the armature contact surface. First, as the commutator revolves, the scratch will roughen the surface of the brush. On a microscopic scale, this increases the amount of arcing between the brush and the commutator bar segments or slip ring. This increased arcing causes a rougher than normal commutator, resulting in faster brush wear, further roughening of the surface of the brush, and a further increase in arcing, which further damages the commutator or slip ring. Secondly, a scratched commutator or slip ring can result in an out-of-round commutator or slip ring, which also causes increased arcing and further damage to brushes and commutator or slip ring. Again on a microscopic scale, when a brush and a scratch interact, the brush is either cocked in its holder, or bounced a short distance radially to the commutator or slip ring. If it is cocked in the brush holder, it will rock or oscillate for a short period of time. As it rocks or oscillates, increased arcing occurs between the edge of the brush further from the commutator or slip ring, wearing both brush and commutator or slip ring. In addition, the resultant rounding of the brush, on a microscopic scale, reduces contact area between the brush and the commutator or slip ring, further accentuating wear to both the commutator or slip ring and the brush. If the brush is deflected vertically, the commutator or slip ring will turn beneath it before it returns (and bounces several times). During this time, arcing will increase, resulting in a small flat spot on the commutator or slip ring. This flat spot will be accentuated during subsequent revolutions of the electric machine, since, even ignoring the effect of the scratch, the commutator or slip ring effectively falls away from the brush when it reaches the flat spot, causing further accentuated arcing, and further wear. And, of course, the brush bounces when recontacting the commutator or slip ring, causing further small flat spots. This semirandom shape causes much increased brush wear, since the brush crumbles slightly each time the brush spring returns it violently to the surface. Therefore, it is important to avoid scratching the commutator or slip ring while installing and removing brushes. Scratching of a commutator or slip ring is much less likely to occur when the brushes may be removed radially to the commutator or slip ring, as in large, stationary motors and generators, operating in an extremely clean environment. However, when a motor must be protected due to its operating environment, or when surrounding structure blocks radial access to the motor, axial movement of either brushes or commutator or slip ring must occur when brushes are being renewed, whether by use of applicants invention, or by conventional disassembly of the motor. One structure for a brush holder which may be removed axially is disclosed in U.S. Pat. No. 3,450,915, issued July 17, 1969, to Zumsteg entitled BRUSH ARRANGEMENT FOR DIRECT CURRENT DYNAMO-ELECTRIC MACHINE. There, brush holders are secured to a ring which is removably attached to the motor end plate. The end plate of the machine is provided with openings through which the brush holders and brushes protrude. As the ring is removed, a spring loaded pin acts to apply an end pressure against a brushes in the brush holder to keep the brushes from falling out of the brush holder. Conversely, this spring loaded pin will hold the brushes in the brush holder, out of contact with the commutator, as new brushes are being inserted from the end of the motor, until the brushes are almost in place, at which time the spring loaded pin releases pressure against the brushes, releasing the brushes. Therefore, for the last increment of movement, the brushes will slide axially along a commutator segment or segments, and depending on brush material, may do microscopic scratch-type damage to the commutator, with the possible results described above, or may damage a brush by scraping it across the sharp commutator edge. In this regard, it should be noted that new brushes have sharper edges than used brushes, and are more likely to cause damage to the commutator. Also, and most importantly, with the structure disclosed by Zumsteg, it is impossible to cock or tilt the brush holder to insure that the hard metal brush holder or brush box itself does not touch the commutator, in addition to the fact that the disclosed structure is inoperative when supporting and surrounding structure crosses the axis of the motor, such as when a motor is used on an industrial forklift truck. In such an application, the task of removing and replacing the motor to renew or inspect the brushes consumes about four hours, in contrast to the brush renewal time of about 30 minutes using applicant's invention.
In addition, brushes may be themselves damaged during insertion and removal. Allowing brushes to contact the commutator or, slip ring before fully in place drags the last brush over the sharp corner of the commutator or slip ring, possibly breaking the brush. Upon removal, if not restrained, a brush will snap downward as it clears the commutator or slip ring and may smash against other structure, or may simply protrude too far, and be easily accidentally broken. The instant invention overcomes such disadvantages of known structures for brush holders.
Also, it is desirable to maintain the surface of a brush which slides upon an armature contact surface in compression, to insure firm contact. The amount of pressure on a brush may determine its rate of wear and useful life. While frictional forces increase as pressure upon a brush increases, this does not imply that low brush pressure corresponds to long brush life. To the contrary, low brush pressure results in chattering and increased arcing, and rapid brush wear, by some of the mechanisms described above. Therefore, pressure upon brushes should be maintained at or near a value experimentally determined to result in a minimum brush wear rate. One prior attempt to do this utilized cumbersome movable pivoted brush holders in the shape of a lever arm and torsion springs with free ends insertable in notches in the lever arm body. This approach may be usable with large dynamoelectric machines, where the brush holder assembly is freely and easily accessible, and where there are no space or size limitations.
Where space permits, a linear spring which coils up when released may be used to provide brush pressure. A free end of this spring is anchored to the brush box near the armature contact surface, pulling the brush towards the armature as the brush wears and the spring coils up and presses its furthermost surface. This is a bulky arrangement, best suited for large stationary generators and motors, so that extremely long brushes can be used, for long brush life.
Conventionally, particularly in mobile application, where space and weight restrictions apply, a spiral-wound spring, with a free end in the form of a cantilevered lever arm, provides a somewhat constant force upon a brush, but makes no provision for adjusting pressure upon a brush in accordance with experimentally determined requirements, or even for compensating for the decrease in pressure as the spiral spring unwinds.
The instant invention overcomes these and other deficiencies of the prior art.